1. Field of the Invention
The present invention relates to methods and apparatus for measuring the solids concentration of a fluid suspension; and more particularly, to methods and apparatus for monitoring and controlling the concentration of magnetic particles in a magnetorheological fluid being used in a magnetorheological finishing apparatus.
2. Discussion of the Related Art
It is well known in the art of finishing and polishing surfaces to use, as a finishing agent, particulate fluid suspensions having magnetorheological properties. Such fluids, known as magnetorheological fluids (MR fluids), comprise magnetically soft particles, such as iron carbonyl, which can become oriented and magnetically linked into fibrils in the presence of a superimposed magnetic field, thereby increasing the apparent viscosity of the fluid by many orders of magnitude. Such increase is known as magnetic “stiffening” of the MR fluid. Non-stiffened, or magnetically relaxed, MR fluid can be stored and pumped as a low-viscosity fluid, having a viscosity  typically of about 50 cp or less, then stiffened to a semi-rigid paste of about 105 cp or more in a magnetic work zone for finishing or polishing, then relaxed again outside the work zone for collection, reconditioning, and reuse. Apparatus and methods for magnetorheological finishing and for delivery of MR fluids are disclosed in, for example, U.S. Pat. No. 5,951,369 issued Sep. 14, 1999 ('369) and No. 5,971,835 issued Oct. 26, 1999 ('835), and Published U.S. Patent Application No. 20020177392 ('392), all to Kordonski et al., the relevant disclosures of which are herein incorporated by reference.
MR fluid finishing apparatus typically includes a fluid delivery system (FDS) for dispensing MR fluid onto a rotating carrier surface, whereon the fluid is carried into and out of a work zone formed against the surface to be finished. MR fluid is a relatively unstable suspension because the magnetic particles tend readily to agglomerate and to settle out of suspension and thereby stagnate. Thus, a primary concern in configuring an FDS for MR fluid is keeping the fluid relatively homogeneous in the system, and very highly homogeneous at the point of dispensing into the work zone. An FDS must receive spent fluid from the work zone, recondition the fluid for reuse as by adjusting the temperature and viscosity, rehomogenize the adjusted fluid, and redispense the fluid into the work zone at a controlled flow rate. A suitable prior art FDS is disclosed in the '392 Publication.
MR fluid is a mixture of iron particles and carrier fluid, which, for example, can be water. The iron particles and water must be mixed and maintained in a certain proportion to maintain required magnetorheological and material removal properties  throughout a polishing or finishing run, which can last up to several hundred hours. Evaporation of water causes a natural rise in solids concentration of the MR fluid as the fluid passes through the work zone, and also in the collection/mixing reservoir. To maintain a consistent solids concentration, water must be added to the fluid in the collection/mixing reservoir.
Preferably, some real-time means is provided to measure (monitor) iron particle concentration in the MR fluid. For example, in magnetorheological determinisitic finishing (MRF), the concentration of iron particles must be precisely controlled in real time to provide the required stability and predictability of the material removal function.
One known approach to monitoring concentration is to continuously measure slurry viscosity, which is proportional to the solids concentration. For example, our '369 patent discloses a method wherein a flow rate and a pressure drop are measured along a capillary tube, and viscosity and concentration are inferred therefrom. Although this method can yield excellent results, several problems can be encountered. Pressure and flow rate must both be measured, and each may be subject to error. Temperature control is required. A relatively long (100×diameter) capillary tube is required to provide the needed accuracy, and changes in capillary tube internal geometry (clogging, sedimentation, wear) can lead to erroneous values. The apparent viscosity of MR fluids, being non-Newtonian, is a function of shear rate, which requires special means to correct measurements when flow rate changes.
Another known approach is disclosed in U.S. Pat. No. 6,650,108, wherein concentration of iron particles in MR fluid  is inferred from measurement of fluid inductance and density. Again, several problems can be encountered in using this approach. The measurements of inductance and density have low sensitivity, meaning that the inferred concentration value has a large margin of error. The inductance-measuring coil can cause structuring of the MR fluid, and magnetic material may sediment on the walls of measuring cell, both resulting in erroneous concentration values. Density measurements using the Coriolis method are complex and expensive. Finally, the disclosure of this patent does not teach how to control and adjust concentration but attempts only to monitor actual values.
What is needed is an improved method and apparatus for measuring and controlling MR fluid solids concentration,                wherein determined values are not sensitive to flow rate and temperature of the fluid, are not sensitive to variation in pipe geometry, and do not require a long and special capillary tube, and        wherein the method and apparatus are easily adapted to provide in-line real time measurements and require simple hardware, including only one pressure gauge.        
It is a primary objective of the invention to provide accurate, reliable, and inexpensive continuous, real-time measurement and control of iron particle concentration in a magnetorheological fluid.